A known method for making an emulsion (or foam) is so-called cross-flow emulsification, wherein a fluid for dispersing is forced as dispersed phase through an injection plate with injection channels, while a continuous cross-flow phase of a second fluid is guided at a certain speed, transversely of the outflow openings of the injection channels, over the outlet side of the injection plate. An example of such a known method and associated device is described in European patent application EP 1.197.262. The second fluid flowing past here exerts a shear stress on the first fluid leaving the injection plate, whereby upon reaching a certain size a microdroplet is separated from the first fluid and entrained and absorbed in the second fluid. The size of the thus formed microdroplets is determined partly by the speed of the second fluid that is flowing past and the nature of both fluids. Microdroplets are thus formed with a varying diameter of typically between 2 and 20 times the effective diameter of the injection channel in the injection plate. It is noted here that where mention is made in the present application of an effective radius or diameter of a channel, this is understood to mean the radius or diameter of an imaginary, perfectly round reference channel of a size such that an equal inflow resistance to the relevant fluid is encountered. In order to enhance shearing of microdroplets by the second fluid, use is made in the known device of injection channels with a non-round and non-square cross-section in order to thus create an unstable boundary surface between the dispersed phase of the first medium and the continuous phase of the second medium at the outflow opening of the injection channel.
It is found desirable for an increasing number of applications that the microdroplets formed with the device are very fine and moreover have a mutually almost equal size. These are for instance microdroplets with a diameter of typically one tenth of a micrometer and several tens of micrometers which are all at least practically of equal size. Such very small, almost mono-dispersed microdroplets result in for instance a great improvement in the stability of an emulsion (oil/water, water/oil). The texture and rheology of many foams also improves if very small and equal gas microbubbles are incorporated in this foam. This latter is found to be particularly important in the dairy industry, wherein light products are in increasingly great demand and optionally multiple emulsions open avenues to new products and product groups.
The known device and method have the drawback that the droplet size depends on more or less chance process parameters and is thereby not fixed but, on the contrary, varies relatively widely within the given limits. In the known device and method for forming the microdroplets a cross-flow of a second fluid on the outlet side of the injection plate is moreover essential. Realizing such a cross-flow of the second fluid is sometimes found to be time-consuming in practice.